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The Feynman diagram for the beta decay of a neutron into a proton, electron, and electron antineutrino via an intermediate heavy W- boson

It is one of three major types of radioactivity (the other two being alpha decay and gamma decay), and the only one that involves the transmutation of a subatomic particle. It is an example of a "weak interaction". The commonest radioactive decay involving the weak interaction is the transmutation of a neutron into a proton, with emission of an electron. Before the structure of the nucleus was understood, this emission of negative particles was observed, and they were called "beta particles" or "beta rays". The beta particles were later determined to be just electrons.

There are other, much rarer, radioactive decays involving the weak interaction. One is the "beta-plus" decay, in which a proton turns into a neutron, emitting a positron (antielectron) and an electron neutrino. This action, by itself, is forbidden by conservation of energy, so it can only occur in nuclei that have unusual energy configurations. Another is the "electron capture" event, in which a proton captures one of the electrons circling the nucleus, turns into a neutron, and emits an electron neutrino. While this is not energetically unbalanced, it requires that an electron get very close to the nucleus, which is extremely rare under the rules of quantum mechanics.